Sewing machines are known which form stitches of a zig-zag pattern and a lettering pattern by swinging a needle bar from side to side relative to a cloth feeding direction. According to such sewing machines, the needle bar swings from side to side in accordance with an up-and-down motion of the needle bar. Hence, the needle bar is located at the right and left needle drop positions.
Sewing machines are provided with a needle drop hole in a stage on which a cloth is placed. In the case of sewing machines that swing the needle bar from side to side, the needle drop hole extends in an orthogonal direction to the cloth feeding direction. The needle bar can swing in the direction in which the needle drop hole extends. By swinging the needle bar, a needle can fall to an arbitrary location within the needle drop hole.
In this case, a position of the needle relative to the needle drop hole is called abase line. For example, the position of the needle when the needle falls to the center in the needle drop hole is called a center base line. The position of the needle when the needle falls to the left side in the needle drop hole is called a left base line. The position of the needle when the needle falls to the right side in the needle drop hole is called a right base line. In this case, the right and left sides are defined with reference to the cloth feeding direction.
Sewing machines include a swing mechanism that swings the needle bar from side to side relative to the cloth feeding direction. This swing mechanism changes the base line of the needle. The swing mechanism can change the position of the needle from the left base line to the right base line, and from the right base line to the left base line. By changing the base line in this way, stitches for a zig-zag shape can be formed.
In addition, the swing mechanism can adjust the width of the swing of the needle bar from side to side. By adjusting the swing width of the needle bar, the width of zig-zag sewing can be adjusted. Still further, such sewing machines can realize complex sewing, such as whip stitch, pattern stitch, and letter stitch.
When, however, wide zig-zag sewing or pattern sewing is performed, the swing of the needle bar by the swing mechanism becomes large. In this case, the pitch between the needle drop position at the center base line and the needle drop positions at the right and left needle drop positions increases. According to typical sewing machines, even if the base line is changed, the position of a shuttle that retains thereinside a lower thread does remains the same. In this case, when the base line of the needle changes, the relative positional relationship between the needle and the shuttle changes. That is, the interaction between the needle and the shuttle becomes out of synchronization.
The needle and the shuttle are adjustable so as to form a stitch with a certain margin range. When, however, this non-synchronization in the interaction occurs beyond the margin range, it becomes difficult to form a stitch. Hence, according to conventional sewing machines, the amount of swing of the needle bar is limited within a range that still enables the non-synchronization of the interaction between the needle and the shuttle to form a stitch.
Conversely, for example, JP H01-42229 A discloses a scheme of causing a phase shifting when a rotational motion is converted into a linear motion by utilizing a positional change of the needle bar caused by the swing of the needle bar, and controlling the timing of the up-and-down motion of the needle bar, thereby reducing the non-synchronization of the interaction between the needle and the shuttle while increasing the amount of swing of the needle bar.
According to the scheme disclosed in JP H01-42229 A, however, the positional relationship among respective components that construct a sewing machine is significantly limited. Hence, the scheme of JP H01-42229 A is applicable to industrial sewing machines, but in view of the work positional relationship of normal domestic sewing machines, sewing machines become zig-zag sewing machines that move the needle bar back and forth relative to a user. Hence, it is not practical to apply the above scheme to normal domestic sewing machines.